The invention relates to an apparatus and a method for collecting sheets into sets.
In particular, in the context of machine assembly of mail items, it is known to use an apparatus having:                a main supply station equipped with a sheet holder for storing a stack of sheets, a separation system for separating sheets piece by piece from the sheet holder, and a sheet supply track for conveying separated sheets to the conveyor track; and        at least two insert supply stations, each equipped with a sheet holder for storing a stack of sheets, a separation system for separating sheets piece by piece from the sheet holder, and a sheet supply track for conveying separated sheets to the conveyor track.        
A first one of the insert supply stations is then equipped for supplying sheets to a first position on the conveyor track downstream of the main supply station and a second one of the attachment supply stations is arranged for supplying sheets to a second position on the conveyor track downstream of the first position on the conveyor track.
The sheets can be, for instance, address carriers, letters, brochures, reply cards, prepaid envelopes, carriers with a plastic card, etc.
In practice, it frequently happens that in assembling very large numbers of sets, for the purpose of supplying inserts to main documents coming from the main supply station, a switch is made from one of the insert supply stations to another one of the insert supply stations, both insert supply stations being initially loaded with inserts of the same type.
In order to prevent loss of production, it is desirable that this can be done automatically and without stopping the apparatus, in response to (imminent) emptiness of the insert supply station from which inserts are being added. A problem here is that if the insert supply station that is to take over the supply of documents from an emptied insert station is situated upstream of the emptied insert station, there is at least one main document or at least one set of main documents to which no insert document can be added without stopping the machine and executing such addition manually. This is disadvantageous in particular if the main documents, as is often the case, are each unique documents destined for a particular recipient.